This invention relates to a photometric device such as colormeter for photography, illuminancemeter, and usual colormeter, and also to a light receiving section provided in a photometric device.
Conventionally, photometric devices such as colormeter for photography, illuminancemeter and usual colormeter have been known. Such a photometric device is required to have a characteristics according to its application. Particularly, it is necessary to design the photometric device such that it has a specified oblique incidence angle characteristics which is a common characteristics among photometric devices.
This oblique incidence angle characteristics is described with reference to FIG. 9. A photometric device 111 is provided with a light receiving section 112, whose center O is so arranged as to coincide with Z-axis of X, Y, Z-orthogonal coordinate systems. On this center O is incident an incident light L as a measurement object. Assuming that an angle formed by the incident light L and the Z-axis is a latitude and an angle formed by the incident light L and an XZ-plane on an XY-plane is a longitude, an oblique incidence angle is expressed by an incidence angle .alpha. in the longitudinal direction and an incidence angle .beta. in the latitudinal direction (the incidence angle .alpha. is an angle formed between the incident light L and the Z-axis and the incidence angle .beta. is an angle formed between a plane including the incident light L and the Z-axis and the XZ-plane). The oblique incidence angle characteristics refers to an output dependency of the light receiving section on the incidence angles .alpha. and .beta..
For example, in the case that the photometric device is an illuminancemeter, the oblique incidence angle characteristics in relation to the incidence angle .alpha. is required to satisfy the so-called cosine characteristics standardized by the Japanese Industrial Standards (JIS C 1609) or the like. Further, the oblique incidence angle characteristics in relation to the incidence angle .beta. needs to be designed to eliminate an angular dependency lest measurement vales should vary depending upon how the apparatus is installed (orientation of the light receiving section).
Of course, in the colormeter for photography and usual colormeter other than the illuminancemeter, the oblique incidence angle characteristics in relation to the incidence angle .beta. also need to be similarly designed to eliminate the angular dependency.
In order to attain the oblique incidence angle characteristics in relation to the incidence angle .alpha. as above, a milky acrylic plate or the like diffusing member is placed on the front surface of the light receiving section to sufficiently diffuse the incident light. Further, the diffusing member is shaped to have a semispherical surface and/or the shape of a diffusing member holder around a window for the light receiving section 12 is devised.
Further, as a photometric device with a single light receiving section having a plurality of light receiving elements therein, a colormeter for photography has been, for example, known.
In the colormeter for photography, it is necessary to arrange three light receiving elements (light receiving elements having sensitivities in three wavelength ranges of blue, green and red). FIGS. 10A and 10B show a construction of a colormeter for photography, wherein FIG. 10A is a front view and FIG. 10B is a section taken along the Y-axis. In this colormeter 111, light receiving elements 113, 114, 115 having sensitivities to the respective wavelength ranges of red, green and blue are circumferentially arranged about the center O of the light receiving section 112. FIG. 10A shows a state where a diffusing member 116 of the light receiving section 112 is removed. The diffusing member is a planar member as shown in FIG. 10B.
If all these light receiving elements can be arranged in the center of the light receiving section, they are symmetrical with respect to the optical axis of the light receiving section 112 for a reason described later. Even if the diffusing member placed on the front surfaces of the light receiving elements were spherical, the oblique incidence angle dependency in the longitudinal direction could be, in principle, eliminated. However, since it is, in reality, physically difficult, the light receiving elements have to be circumferentially arranged about the center O of the light receiving section. Accordingly, the diffusing member is not allowed to have a curved surface, i.e., it has a planar surface.
In other words, the diffusing member has to have a planar surface so as to eliminate the oblique incidence angle dependency in the longitudinal direction even for the light receiving element arranged in a position displaced from the center O. As a result, the oblique incidence angle characteristics in the longitudinal direction is automatically determined by the material and thickness of the diffusing member.
Next, the illuminancemeter is described. The spectral sensitivity of the light receiving section of the illuminancemeter is required to agree with a relative luminous sensitivity characteristics V(.lambda.) of the CIE (Commission Internationale de l'Eclairage) standard observer. In conventional illuminancemeters, a filter method has been adopted according to which the spectral sensitivity of the light receiving section is made to maximally approximate to the relative luminous sensitivity characteristics V(.lambda.) by combining light receiving elements such as silicone photosensors with a color glass filter, interference filter, film filter (gelatin filter or acetate filter) or like filter.
However, the aforementioned filter method has a problem that, if an attempt is made to maximally approximate the spectral sensitivity of the light receiving section to V(.lambda.), the number of filters increases and the amount of light reaching the light receiving elements is reduced. If the number of the filters are reduced in order to increase the light amount, a degree of approximation to V(.lambda.) is disadvantageously degraded Further, even if there are variations in the respective characteristics of the light receiving elements, there is no problem if variation in the total spectral characteristics by combination of the filters and the light receiving elements lies within a permissible range. However, a strict administering is necessary so that the respective parts lie within such permissible ranges.
In order to overcome these problems, the following technique has been used in the prior art. Specifically, the light receiving section is constructed by combining a plurality of light receiving elements and filters having mutually different spectral transmittance characteristics, and the outputs of the light receiving elements are increased or decreased later by multiplying weight factors to attain a desired spectral sensitivity. Such a technique for synthesizing the desired spectral sensitivity using a plurality of light receiving elements is well-known not only in the illuminancemeter art but also in the art which seeks to attain a desired spectral sensitivity. For example, a colormeter adopting a three element method, taking advantage of a property that, out of three spectral sensitivities of the CIE spectral tristimulus values x(.lambda.), y(.lambda.), z(.lambda.), the shape of the short-wavelength side one of two concave curves of sensitivity in a visible region is analogous to that of z(.lambda.), equivalently calculates a total output of x(.lambda.) by multiplying the output of the light receiving element of z(.lambda.) by a specific factor and adding this multiplied output to the output of the light receiving element having a sensitivity of the long wavelength side of x(.lambda.). Further, in order to improve the accuracy of the spectral sensitivity characteristics of a colormeter, a proposal has been made to add and/or subtract the outputs of the three spectral stimulus values to and/or from each other (Japanese Unexamined Patent Publication No. 2(HEI)-45718). Another proposal has also been made to apply this concept to the standard relative luminous efficiency (Japanese Unexamined Patent Publication No. 7(HEI)-318423).
In the illuminancemeter of the spectral sensitivity synthesizing type using a plurality of light receiving elements as well, the problem arises that it is physically difficult to arrange all light receiving elements about the center O, similar to the aforementioned colormeter for photography.
In the aforementioned colormeter for photography, the oblique incidence angle characteristics in the longitudinal direction is automatically determined by the material and thickness of the diffusing member since the planar diffusing member is used in the light receiving window. Accordingly, the characteristics which should originally be inherent cannot be attained.
Specifically, the colormeter for photography is a device for analyzing the illumination condition of the same object in the same photographic scene (illumination condition) as a general incident light type exposure meter for photography. Accordingly, the oblique incidence angle characteristics of the both should originally agree with each other. However, the oblique incidence angle characteristics of the conventional colormeter for photography and that of the incident light type exposure meter could not agree with each other as described in detail below.
Generally in the incident light type exposure meter for photography, a light receiving sphere as a diffusing member having a semispherical surface is mounted on the light receiving section in the case that a three-dimensional object such as a person is photographed in an actual photographic scene. A measurement is performed by setting the light receiving section such that it faces a lens of a camera from the position of the object.
The oblique incidence angle characteristics in the latitudinal direction .alpha. when the light receiving sphere is mounted on the light receiving section of the incident light type exposure meter is so designed as to display an oblique incidence angle characteristics of the so-called cardioid shape as shown in FIG. 12. This characteristics considers lights which influence the photographing such as light La propagating toward a camera 201 from a side surface of an object 200 and light Lb propagating toward the camera 201 from a position obliquely behind the object 200.
The incident light type exposure meter and the colormeter for photography are adapted to measure and analyze the amount of light and the quality of illumination, receptively, in the same photographic scene (illumination condition). Accordingly, the oblique incidence angle characteristics should originally agree with each other. Specifically, in the colormeter for photography as well, light illuminating the object should be collectively measured in an actual photographic scene, using the light receiving sphere. However, in this colormeter, a planar diffusing member as a diffusing member has to be used for the following reason.
Specifically, the oblique incidence angle characteristics in the latitudinal direction of the colormeter for photography is automatically determined by the characteristics of the planar diffusing member, hence the oblique incidence angle in the latitudinal direction of "cardioid shape" as in the usual exposure meter cannot be realized. In other words, the oblique incidence angle characteristics of the conventional colormeter for photography is narrower than that of the general incident light type exposure meter and, thus, the illumination light rays from the lateral side of the object and from the obliquely behind of the object are evaluated with low degrees. Conversely, if a diffusing member having a spherical surface is used to realize the oblique incidence angle characteristics of "cardioid shape", balance in the incident light amount on the respective light receiving elements of red, green and blue is lost depending on the incident direction in the longitudinal direction, thereby making a deviation of analysis value larger.
FIG. 11 shows an example of oblique incidence angle characteristics of a light receiving element arranged outside the optical axis, for example, of the green light receiving element 114 in FIG. 10, in the longitudinal direction .beta. with respect to oblique incident light when the diffusing member has a spherical surface. Ideally, the oblique incidence angle characteristics should be a circular characteristics about an origin of the coordinate systems where sensitivity is constant independently of the longitude as characteristics N shown by the broken line in FIG. 11. However, in reality, the characteristics is displaced with respect to the origin like characteristics M shown by the solid line in FIG. 11 due to the angular dependency in the longitudinal direction .beta.. An effort is being given to improve a degree of diffusion by devising the characteristics of the diffusing member arranged on the front surface of the light receiving elements so that the oblique incident light can also be uniformly incident on the respective light receiving elements. However, if the degree of diffusion is increased to the extent that the oblique incidence angle dependency in the longitudinal direction becomes negligible, the level of amount of light incident on the light receiving elements is reduced. Thus, the oblique incidence angle dependency in the longitudinal direction cannot completely be eliminated and the elimination of this dependency should, at present, be compromised at a certain degree.